A plasma CVD apparatus which forms a thin film such as an insulating film on a semiconductor wafer using plasma vapor phase excitation has been conventionally used in semiconductor device manufacturing process. This plasma CVD apparatus supplies material gas which consists of elements constituting a thin film onto the semiconductor wafer and forms a desired thin film by a vapor phase or a chemical reaction on the surface of the semiconductor wafer. Plasma discharge is used to excite gas molecules.
FIG. 4 shows the configuration of a conventional plasma CVD device. In FIG. 4, a reaction container 10 is a container which has an evacuated interior and which allows an insulating film to be formed on a semiconductor wafer 19 having a diameter of 12 inches. A nozzle 11 which emits Ar gas, a nozzle 12 which emits O2 gas and a nozzle 13 which emits SiH4 gas which serves as the material gas explained above are provided on the inner side face of the reaction container 10.
An RF electrode 14 is provided on the upper section of the reaction container 10 and connected to a high frequency power supply 15. This RF electrode 14 generates a high frequency electric field to deposit SiOX on the semiconductor wafer 19. As shown in FIG. 5A, during the vapor deposition, an insulating film 19b is formed to cover wirings 19a formed on the semiconductor wafer 19. At this moment, however, the insulating film 19b does not completely reach gaps between the wirings 19a. In FIG. 5A, an RF input is a high frequency input from the RF electrode 14. In addition, the RF power of the RF electrode 14 is set at, for example, 3 kW.
A support base 16 is provided in the reaction container 10 and supports the semiconductor wafer 19 by an electrostatic force. A bias electrode 17 is embedded in the support base 16 so as to be opposed to the RF electrode 14 and is connected to a high frequency power supply 18.
The bias electrode 17 applies a bias so as to draw ionized Ar+ into the semiconductor wafer 19. The ionized Ar+ etches the insulating film 19b deposited on the upper corner sections of the wirings 19a. In this instance, therefore, the upper sections of the gaps between the wirings 19a are always opened, making it possible to evaporate the insulating film 19b compactly into the gaps between the wirings 19a. In FIG. 5B, an LF input is a bias input from the bias electrode 17. The bias power of the bias electrode 17 is, for example, 1 kW.
According to the configuration explained above, the Ar gas, the O2 gas and the SiH4 gas are constantly emitted from the nozzles 11, 12 and 13 into the reaction container 10, respectively, as can be seen from “B”, “C” and “E” shown in FIG. 6. Likewise, the high frequency power supplies 15 and 18 are constantly connected to the RF electrode 14 and the bias electrode 17, respectively. That is, as can be seen from “A” and “D” shown in FIG. 6, the RF electrode 14 and the bias electrode 17 are kept in an RF input (high frequency input) state and an LF input (bias input) state, respectively. Therefore, vapor deposition due to the RF input and sputtering due to the LF input are simultaneously carried out in the reaction container 10.
In other words, as shown in FIG. 5B, the insulating film 19b which consists of SiH4 is evaporated on the surface of the semiconductor wafer 19 and sputtering is carried out so that Ar+ is drawn into the semiconductor wafer 19 side. As a result of this sputtering, the excess insulating film 19b is scraped off and the insulating film 19b spreads into the gaps between the wirings 19a.
The conventional plasma CVD apparatus draws Ar+ into the semiconductor wafer 19 by applying a bias thereto from the bias electrode 17 shown in FIG. 4. However, when the bias is applied, hydrogen existing in the reaction container 10 is also drawn into the semiconductor wafer 19. FIG. 7A is a view which shows a relationship between the mass number of an element and current (drawn-in quantity) when bias is OFF. FIG. 7B is a view which shows a relationship between the mass number of an element and current (drawn-in quantity) when bias is ON. The mass number of an element=2 corresponds to that of hydrogen molecules (H2).
The quantity of hydrogen which is drawn into the semiconductor wafer 19 rapidly increases when bias is OFF and ON. In this instance, deterioration due to reduction with hydrogen occurs in the semiconductor wafer 19, which adversely influences device characteristic. If the semiconductor wafer 19 is made of a ferroelectric material, in particular, the P(polarization)-V(applied voltage) characteristic of the semiconductor wafer 19 (semiconductor device) deteriorates as shown in FIG. 8. That is, before film formation, the P-V characteristic has an ordered before-film-formation hysteresis loop 30. After film formation, the P-V characteristic has a disordered after-film-formation hysteresis loop 31.